Field of the Invention
The present invention relates to a current transformer with a current sensor for determining an effective value of a conductor current. The invention furthermore concerns a method for converting a conductor current flowing in an electrical conductor into an output signal.
Description of Related Art
Current transformers are used to measure a usually large conductor current flowing through a conductor. They deliver an output signal—either an output voltage signal or an output current signal—which is linear to the conductor current being measured.
Current transformers are known in which the conductor circuit being measured is coupled to the output circuit by means of a magnetically conducting (iron) core through which both circuits pass. As the current varies in the circuit being measured, the magnetic field in the core produced by the conductor current varies. This induces a voltage in the output circuit which is detected. However, the current detection by induction only allows for the detecting of alternating currents in the circuit being measured.
In contrast to this, direct currents can also be detected with current transformers which work with current sensors, such as a Hall effect sensor or a shunt resistor. A Hall effect sensor for example is positioned in a magnetic field produced by the conductor current being measured, and the magnetic flux of the magnetic field is detected. After an offset correction which may be necessary, an output voltage put out by the Hall effect sensor is proportional to the magnetic flux and thus to the conductor current. Such a measurement is done, for example, with a clip-on ammeter, which encloses the conductor through which the current is flowing. In the case of a shunt resistor used as current sensor, the conductor current being measured flows directly through the shunt, and the voltage drop across it is measured.
The output voltage delivered by the current sensor is a signed value, where the sign indicates the current direction. In current transformers which are designed to measure alternating currents, an effective value (RMS—Root Mean Square) of the output voltage of the current sensor is formed and thus an output signal is put out which is linear to the effective value of the conductor current.
In the current transformers of the prior art, owing to the forming of the effective value, such current transformers are unipolar in design. Therefore, the measurement of a direct current (DC—Direct Current) IDC,LOAD as a conductor current always furnishes a positive output signal, regardless of the direction of the conductor current.
In addition, current transformers are known for measurement of direct currents in which the direction of the direct current is represented in the output signal. Such transformers are also known as bipolar current transformers. However, with these transformers a measurement of alternating currents is not possible. Depending on the reaction time of the current transformer, an alternating current flowing in the conductor either results in an output signal corresponding to a current strength of zero or an output signal alternating with the frequency of the alternating current.
The present invention was developed to create a current transformer with a current sensor which is suitable both for a current transformation of an input-side alternating current, and an input-side direct current as well as a hybrid current, and in which the direction of the input-side direct current is identifiable.